A microporous polyolefin film prepared through a stretching process has been widely used as a separator for various batteries, a filter for separation, and a membrane for microfiltration, due to its chemical stability and excellent physical properties. A separator for a secondary battery has high ionic conductivity through inner pores, together with a spatial separation function between a cathode and an anode.
A lithium secondary battery has high power/capacity density, but it employs an organic electrolyte. This organic electrolyte is decomposed due to overheating generated from abnormal behavior of the battery, and thus, the battery is ignited and exploded.
The microporous polyolefin film of the existing battery sufficiently performed a role of ensuring safety of the battery by a shutdown function, by which micropores are closed by thermal runaway generated from abnormal behavior of the battery. In recent, power/capacity density of the secondary battery are considerably increased due to high capacity/high power of the secondary battery, and thus the amount of heat generated at the time of abnormal behavior of the battery is increased largely. For this reason, the temperature of the battery instantly rises above the shutdown temperature of the microporous polyolefin film at the time of abnormal behavior of the battery, and thus, a separator is thermally melted and the film is shrunk. In this case, an electric short between electrodes occurs and an energy potential difference drops in an instant, and thus energy is intensively released from a portion where the electric short occurs. As a result, this causes an accident in which the battery is ignited, and further, exploded. Since this secondary battery with high capacity/high power is more widely applicable, the possibility of an accident injuring or damaging persons, buildings, properties and the like around the battery is more likely to occur. Therefore, the development of a separator having excellent thermostability at high temperature is being urgently required.
Japanese Patent Laid-Open Publication No. 2005-209570 discloses a multilayer type separator having an improved thermostability, which is manufactured by forming a coating layer on a polyolefin separator using a resin having high thermastabilty at 200° C. alone, in order to improve thermostability at high temperature. However, there is a disadvantage in that shrinkage of the separator generated at high temperature can not be sufficiently prevented because of using the high thermostable polymer resin alone. Further, since a resist layer capable of suppressing an electric short generated in the battery and improving a resistance force to foreign materials such as dendrites is not provided, there is a limit in manufacturing a battery having improved safety.
Although addition of inorganic matter is suggested in order to solve the instability problem caused by using only the thermostable resin, there is a problem in that the resin used is melted in an electrolyte liquid in most cases. Here, the high thermostable polymer resin, which supports the inorganic matter at high temperature, is melted to allow an inorganic layer to be separated, resulting in an unstable and irregular surface structure of the separator, and thus, power is reduced and stability is deteriorated.
Japanese Patent Laid-Open Publication No. 2007-273443 describes a method of coating on one surface of a base layer with a high thermostable polymer solution and then extracting a plasticizer, in order to compensate for a reduction in permeability due to formation of a coating layer. However, this method has disadvantages in that, although the high thermostable resin is used, the coating layer is easily separated from the base layer because adhesion between the coating layer and the base layer is hard due to the existence of the plasticizer on the surface of the base layer, uniformity among products is decreased because the coating layer is dissolved by the influence of a solvent used for extracting the plasticizer, and the stability is not excellent because a counter reaction may occur in the battery due to the remaining plasticizer. Furthermore, this method is not effectively because an extraction device becomes contaminated due to the complexity of the process and separation of the inorganic matter.